1. Field of the Invention
The present invention relates to a power supply device. More particularly, the present invention relates to a power supply device and a single photo-coupler control circuit thereof.
2. Description of the Related Art
With an increasing demand for energy, saving energy has become an unwritten goal for the design of products. In many types of electronic products, power wastage in the standby mode is one big problem that attracts the attention of engineers and designers. At present, various environment protection agencies and governmental organizations would sit together from time to time to lay down some guidelines and rules for reducing energy waste in the standby mode. For example, in a notebook computer, a power source having higher conversion efficiency and lower standby power consumption is always demanded. For a switch mode power source, the switching loss is the principle power loss when operating in the standby mode. The conventional methods of reducing power loss in the power source when operating in the standby mode mainly depend on reducing the loss in switching the main circuit. However, to attain a high power conversion efficiency for a high-power notebook computer, for example, the main circuit must be optimized when fully loaded while the power rating of various main switching elements must be high. Accordingly, power source with a high power rating can hardly be optimized in the standby mode. Furthermore, a power source having a higher power rating will have a larger switching loss under the same switching frequency. Consequently, for a high-power notebook computer, the conventional power source design can hardly reduce power loss in the standby mode.
In recent years, a new method of reducing power source switching loss when operating in the standby mode is provided. In this method, a main converter and an auxiliary converter connected in parallel and operating in the pulse width modulation (PWM) mode are used. FIG. 1 is a block diagram of a conventional power supply device with a main and an auxiliary converter. As shown in FIG. 1, the rectifier 110 rectifies an AC current from an AC power source to output a DC voltage. According to the control signal provided by the controller 122 and the controller 132, the main converter 120 and the auxiliary converter 130 together provide the output power needed by the load LD. The photo-couplers 121 and 131 provide feedback paths for transmitting load signals back to the controllers 122 and 132 respectively while isolating the power source from the load.
In the standby mode, the output power is only provided by the auxiliary converter 130. When operating with a normal load, most power is provided through the main converter 120 while the auxiliary converter 130 operates in a power-limiting mode (that is, the output power is smaller than a definite fixed value). Thus, a high power conversion efficiency and lower standby power loss are simultaneously achieved when this method is applied to the power source of a high-power notebook computer.
However, according to the conventional feedback design, this type of parallel-connected structure requires two feedback circuits with each feedback circuit (mainly a photo-coupler) consuming considerable standby power. Therefore, the significant power loss for this type of design operating in the standby mode can hardly meet the stringent standby power loss standard. In other words, the conventional technique has at least two major problems, namely, the assignment of power between the two converters and the increase in power loss in the standby mode due to the presence of an additional feedback circuit.